Field of the Invention
The present invention relates to an electrophotographic color printer. It is particularly, but not exclusively, concerned with an electrophotographic color printer capable of achieving full-color images. The present invention also relates to a method of printing using such a printer.
In a single-color electrophotographic printer, a latent image is formed on a photosensitive body by exposing that photosensitive body to a suitably modulated light beam, and then toner material of the color to be printed is applied to the photosensitive body, such that the pattern formed by the toner material corresponds to the latent image on the photosensitive body. This transferring step is also referred as developing the latent image. Subsequently, the toner material on the photosensitive body is transferred to a recording medium (for example, a sheet of paper), and is fixed thereto by a suitable fixing process (for example, heating the toner material to cause it to fuse).
In order to achieve full-color printing, four colors of toner materials are needed. These are cyan C, magenta M, yellow Y and black B. The first three of these (referred to as primary colors) are mixed to provide other colors. For example, magenta M and yellow Y together produce red R, yellow Y and cyan C produce green G, and cyan C and magenta M produce blue B.
In known full-color electrophotographic printers, for example as disclosed in JP-A-2-146065, the full-color image is produced by repeating for each color the steps that are carried out in single-color printing. Thus, for each of the four toner colors, the steps of exposing the photosensitive body, applying colored toner material to the photosensitive body and transferring the toner material image to a recording medium, had to be carried out sequentially. Such printing was slow, and also required precise alignment of the recording medium when the toner materials were each transferred, to ensure correct superposition of the primary color toner materials when it was desired to produce secondary colors.
Proposals have also been made for two-color electrophotographic printing, requiring only a single exposure step for the two colors. In U.S. Pat. No. 4,078,929, there is disclosed a printing method in which a photosensitive body is exposed to three different voltage levels, corresponding to a background voltage level, a first voltage level which is positive relative to the background voltage level, and a second voltage level which is negative relative to the background level. Then, by suitably electrically charging toner materials of two different colors a toner material of one color is applied to the regions of the photosensitive body exposed to the first voltage level, and a toner material of the second color is applied to the regions of the photosensitive body corresponding to the second voltage level. Since only one exposure step is then needed printing of the two colors is relatively fast and alignment of the two colors is ensured.
A development of this technique is disclosed in U.S. Pat. No. 4,731,634, in which a latent image had regions of a third voltage level more negative than the second or background levels. Toner of a first color (black K) is then applied to the regions of the first voltage level, toner material of a second color (cyan C) is applied to the regions having either the second or the third voltage levels, and toner material of a third color (magenta M) is applied to the regions of the third voltage level, by suitably adjusting the bias levels of the means for applying the toner materials to the photosensitive body. Such an arrangement permitted a color image to be formed having black K regions, cyan C regions, and blue B regions where both cyan C and magenta M were transferred.
Finally, U.S. Pat. No. 4,903,048 disclosed simulated color imaging by making use of an optical addictive process. However, in order to produce good colors, a subtractive process must be used which superimposes color materials.